The isotopic composition and atomic weight of ytterbium

The isotopic composition of ytterbium has been measured with high precision using a thermal ionization mass spectrometer, the linearity of which was verified by measuring the isotopically certified reference material for potassium (NIST 985), whose isotopes span a wide range of isotope ratios. The abundance sensitivity of the mass spectrometer in the vicinity of the measured ion beams has been examined to ensure the absence of tailing effects. Particular care was given in ensuring that potential isobaric interferences from erbium, lutetium and hafnium did not affect the measurement of the ytterbium isotopic composition. Gravimetric mixtures of two isotopically enriched isotopes 171Yb and 176Yb were used to calibrate the mass spectrometer, thus enabling the “absolute” isotopic composition of ytterbium to be determined. An accurate determination of the isotopic composition of ytterbium is required in order to calculate its atomic weight, the magnitude of which has remained unchanged since 1934, when it was determined by the Harvard chemical methodology. A number of mass spectrometric measurements of the isotopic composition of ytterbium have previously been undertaken, but none have taken into account linearity and isotope fractionation factors. The isotope abundances (in at.%) of ytterbium measured in this experiment are as follows: 168Yb = 0.1232 ± 0.0004; 170Yb = 2.982 ± 0.006; 171Yb = 14.086 ± 0.020; 172Yb = 21.686 ± 0.019; 173Yb = 16.103 ± 0.009; 174Yb = 32.025 ± 0.012; 176Yb = 12.995 ± 0.0013. These values enable an atomic weight Ar(Yb) = 173.054 ± 0.001 to be calculated as compared to the presently-accepted value of 173.04 ± 0.03, together with the solar system isotope abundances of Yb.